Completely different cars. Here are the biggest differences and would be happy to talk live if you want more info PM me.

1. GT500 - Raw power, very quick and super fast - in a straight line
1a. M3 - slower on the power. Not as "snappy". Way more controlled.
2. GT500 - handling is horrible. Needs $1000 worth of suspension before you can even drive it. It is called wheel hop. Do your research.
2a. M3 - well I guess this is a no brainer. Race car out of the box.
3. GT500 - Almost scary to drive and will bite you if you do not know what you are doing. Almost have to drive on edge until you learn the car.
3a. M3 - Way more fun to drive and you know the car can handle anything, within reason, that you throw at it.
4. GT500 - Interior and appointments are, well, it is a $50K Ford muscle car. You get what you pay for.
4a M3 - It is like a luxury race car. Can take it out on a date dressed up and feel good about it.

Hope that helped. Go borrow one for a day and you will be over the power, speed and G's in a straight line.